In conventional small fire arms, the ammunition is initiated by the percussion of a primer which triggers the firing of the propellant charge of the ammunition. The firing pin of the weapon, activated either by a mechanical or electric means, crushes the primer fixed to the base of the ammunition cartridge. When cased ammunition, is replaced by caseless ammunition, the problem of sealing for the combustive gases inevitably arises. In fact, the firing pin is a projecting mobile element in the weapon chamber where the high pressure may reach 5.10.sup.8 Pa, which makes it difficult to seal.